Recently, the portable terminals such as laptop computer, mobile phone and PDA (Personal Digital Assistance) are widely used. For secondary batteries used for power sources of these portable terminals, nickel hydrogen secondary battery, lithium ion secondary battery or so are heavily used. The portable terminals are rapidly downsized, made thinner, have become lighter, and has higher performance. As a result, the portable terminals are used in various occasions. Also, for the battery, it is required to be downsized, made thinner, to be lighter and to have higher performance.
As for the positive electrode active material which is the constitution material of the lithium ion secondary battery, the active material comprising the transition metal such as iron, manganese, cobalt, chromium and copper or so are used. When the charge-discharge is repeated in the secondary battery using this active material, the transition metal ions elutes out to the electrolytic solution, and as a result, the battery capacity or the cycle characteristic may decline in some cases, which has become a significant problem.
Also, the transition metal eluted from the positive electrode is deposited at the negative electrode surface by reduction; thereby the dendritic metal deposition is formed. As a result, this damages the separator, and the safety of the battery declines which has become a significant issue as well.
The electrodes used in the lithium ion secondary battery usually has a structure wherein the electrode active material layer is stacked on the current collector; and at the electrode active material layer, besides the electrode active material, the polymer binder (it may be referred as “binder” in below) which binds the electrode active materials with each other, and also the electrode active material and the current collector is used. The electrode is usually produced by obtaining the slurry composition by mixing the binder composition wherein the polymer which becomes the binder is dispersed or dissolved with water or the organic solvent or so, with the active material and the conductive agent such as conductive carbon or so if needed; then this slurry composition is coated on the current collector, then dried thereby the electrode is produced.
In the polymer binder, particularly as the polymer binder of the positive electrode, a fluorine based polymer such as polyvinylidene fluoride (PVDF) has been preferably used since it is difficult to dissolve against the organic electrolytic solution.
However, fluorine based polymer such as polyvinylidene fluoride has week adhesive force against the current collector, and there was a problem that battery capacity declines due to the deterioration of the electrical connection between the electrode active material layer and the current collector by repeating the charge-discharge. Also, if the amount of the fluorine based polymer such as polyvinylidene fluoride is increased in order to enhance the adhesive force with the current collector, the internal resistance of the battery increased which causes the capacity to decline.
The patent document 1 proposes to improve the energy density and the cycle characteristic of the battery by using both of the fluorine based polymer such as polyvinylidenefluoride based polymer which has small insulation coating effect against the active material, and acrylonitrile-butadiene rubber or hydrogenated acrylonitrile-butadiene rubber (H-NBR) having excellent adhesiveness.
Also, the patent document 2 proposes to suppress the reduction of the battery capacity even after the large electric current discharge and to suppress the reduction of the battery capacity even after repeating the charging and discharging, by forming the flexible electron conduction network by making the content ratio of the conductive agent in the positive electrode active material layer 1 to 25 times more of the content ratio of the acrylonitrile-butadiene rubber.